Buffers (COMPLETE)

What is a buffer

A solution that resists changes in pH when small amounts of acid or base are added

Components required for a buffer

A weak acid and its conjugate base, or a weak base and its conjugate acid

Example of an acid buffer

Acetic acid and sodium acetate

Example of a basic buffer

Epinephrine and epinephrine HCl

Identify if a solution is a buffer

Must contain a weak acid and its salt OR a weak base and its salt

Identify non-buffer solution

Contains strong acid/base or does not contain conjugate pairs

Why strong acids cannot form buffers

They fully dissociate and cannot establish equilibrium

Why strong bases cannot form buffers

They fully dissociate and cannot establish equilibrium

Write the buffer equation for a weak acid and its salt

pH = pKa + log([salt]/[acid])

Write the buffer equation for a weak base and its salt

pH = pKw – pKb + log([base]/[salt])

When to use acidic Henderson–Hasselbalch equation

When system has a weak acid + conjugate salt

When to use basic Henderson–Hasselbalch equation

When system has a weak base + conjugate salt

What is the pKa of acetic acid used in the example

4.76

In 100 mL of 0.1 M acetic acid, how many moles are present

0.01 moles

Calculate sodium acetate needed for pH 5.2 buffer

0.0273 moles in 100 mL

What happens when acid is added to acetic acid/sodium acetate buffer

Acetate ion neutralizes H₃O⁺ (acid) to form acetic acid

What happens when base is added to acetic acid/sodium acetate buffer

Acetic acid reacts with OH⁻ (base) to form acetate + water

What is the pH of an ephedrine/ephedrine HCl buffer (ratio 10, pKb 4.64)

pH = 10.36

When buffer capacity is highest

When pH = pKa

Buffer capacity definition (β)

Amount of acid or base needed to change pH by one unit per liter of buffer

What is the buffer capacity of blood

0.025 moles/L per pH unit

Why pharmaceutical buffers should have low buffer capacity

To minimize irritation and avoid altering physiological pH

Why parenteral and ophthalmic solutions should be minimally buffered

To prevent irritation due to high buffer capacity

Physiological buffer examples

Bicarbonate, phosphate, and protein buffers

What physiological buffers exist in blood

Hemoglobin/oxyhemoglobin and NaHCO₃/H₂CO₃

What buffers the eye’s lacrimal fluid

Natural buffer system with high capacity around pH 7.4

Why buffer capacity and volume are important

They determine pH changes when added to body fluids

When buffers should not be used

When large capacity may alter physiological pH (e.g., injections)

Preferred pH range for parenteral solutions

Near physiological pH (≈ 7.4)

State one way to reduce irritation from buffers

Avoid large pH differences from physiological fluids

What does a high buffer capacity mean

Buffer strongly resists pH change; may cause irritation in vivo

What does a low buffer capacity mean

Buffer easily changes pH; preferred for injections and ophthalmics

Main buffer system in the body

Carbonic acid/bicarbonate (H₂CO₃/NaHCO₃)

Strong electrolyte definition

Completely dissociates in water (e.g., HCl, NaOH)

Weak electrolyte definition

Partially dissociates (e.g., weak acids, weak bases)

Non-electrolyte definition

Does not dissociate (e.g., sugars, alcohols)

Salt of strong acid + strong base

Produces neutral solution (pH ~ 7)

Salt of strong acid + weak base

Produces acidic solution

Salt of weak acid + strong base

Produces basic solution

Salt of weak acid + weak base

pH depends on Ka vs Kb

Irritation caused by buffer mismatch

pH far from physiological pH causes burning/stinging

Effect of adding strong acid to buffer

Buffer neutralizes H⁺ with minimal pH change

Effect of adding strong base to buffer

Buffer neutralizes OH⁻ with minimal pH change

What determines the pH of a buffer

The ratio of salt to acid (or base to salt)

If acid concentration > salt concentration

pH < pKa (more acidic)

If salt concentration > acid concentration

pH > pKa (more basic)

Buffer region definition

pH = pKa ± 1

Blood pH normal range

7.35–7.45

pKa definition

pH at which an acid is 50% dissociated

Using pKa to choose a buffer

Choose an acid with pKa near target pH

Kw expression

[H⁺][OH⁻] = 1 × 10⁻¹⁴

Relation between pKa and Ka

pKa = –log(Ka)

Relation between pKb and Kb

pKb = –log(Kb)

Relation between pKa and pKb

pKa + pKb = 14